1. Field of the Invention
The present invention relates to a socket for removably mounting an electronic device. More specifically, the present invention relates to a pinch-style support contact configured to establish a reference seating plane for an integrated circuit (IC) package within the socket, as well as provide electrical communication for the IC package and the socket.
2. State of the Art
Testing a semiconductor die often involves establishing an electrical connection between testing equipment and the integrated circuitry of the die. Testing may be performed on an unpackaged semiconductor die that has been singulated from a semiconductor wafer, on a section of semiconductor dice that are still part of the wafer, or on all of the semiconductor dice on a wafer. Moreover, a bare semiconductor die that has undergone packaging may also be tested. One example of such a packaged semiconductor die is a so-called “flip-chip,” wherein discrete conductive elements, such as solder balls, are attached directly to or formed on the bond pads or redistributed bond pads at the ends of electrical traces formed on the active surface of the semiconductor die. The die is then “flipped,” or mounted face down, so that the solder balls may connect with contact members of another device, such as terminal pads of a carrier substrate. Another example is a “chip scale package,” which includes a die along with one or more package elements, such as encapsulating material in the form of thin protective coatings formed of a dielectric material bonded to the active surface, sides and back side of the semiconductor die; in addition, solder balls may be attached to or formed on ends of electrical traces on the active surface of the semiconductor die or directly to the semiconductor die's bond pads through openings in the encapsulating material. A Ball Grid Array (BGA) serves as yet another example that involves even more packaging: the semiconductor die is wire bonded to terminal pads on the top side of an interposer substrate and encapsulated, and solder balls are bonded to electrical traces on the bottom side of the substrate that are electrically connected to the terminal pads.
An electronic device to be tested will hereinafter be referred to as an integrated circuit package, or IC package, regardless of the singulation or packaging state of the semiconductor die or dice that form all or part of the IC package. One method of testing the IC package involves placing the IC package into a socket, which comprises a body with apertures that span through the body. These apertures house contact members that are aligned with electrical terminals of the IC package. For purposes of explanation only, it will be assumed that the terminals of the IC package are solder balls or other discrete conductive elements that protrude from the IC package. Often, the socket includes cover that, when closed, adjusts a slider to actuate arms of the contact members and engage the solder balls of the IC package. Contact members comprising arms which may open and close about solder balls may be referred to as pinch-style contacts. Once the IC package has been inserted, the socket may then be plugged into a printed circuit board (PCB) or other carrier substrate.
One example of a conventional socket with pinch-style contacts used in burn-in tests for electronic packages having BGA terminals is described in U.S. Pat. No. 6,350,138 issued to Atobe et al. (hereinafter “the '138 patent”), on Feb. 26, 2002. The '138 patent discloses, as shown in FIG. 1A and FIG. 1B hereof, a conventional socket 1 including a seating part 5a supporting a BGA package 9 on the periphery of the BGA package body.
The depicted socket 1 comprises a base 2 as the main socket body, a cover 3, a slider 4 mounted on the base 2, which serves as a contact part switching member, and an adaptor 5 mounted on the slider 4. The base 2 may be attached to a PCB (not shown) for testing the BGA package 9. The cover 3 is formed in the shape of a square frame with an opening at the center for the purpose of inserting the BGA package 9. The base 2 and cover 3 are relatively movable toward and away from each other while maintaining a mutually parallel state. Contact members 6 are provided at positions which correspond to solder balls 11 provided on the lower side of the BGA package 9. Each contact member 6 includes a pair of arms 6a, 6b for engaging a solderball 11. The slider 4 includes a lattice-like partition wall capable of moving in a vertical direction, thus engaging the contact members 6, causing the pair of arms of each contact member to open or close. A slider 4 capable of moving in a horizontal direction to engage the contact members 6 is also known in the art. The terminal portions 6c of the contact members 6 provide attachment to the PCB (not shown).
The contact members 6 pass through the base 2 and apertures 4b of the slider 4. The contact arms 6a, 6b include tips 7, located within substantially the same plane. The seating part 5a supporting the BGA package 9 creates a seating plane, the plane of a bottom surface 8 of the IC package body, or substrate 10. One difficulty in the construction of the socket 1 is ensuring that the plane of the contact arm tips 7 is parallel to the seating plane. In addition, the spacing of the plane of the contact arm tips 7 must be a proper distance from the seating plane to ensure reliable electrical connection, as described further hereinbelow. The seating part 5a and the base 2 conventionally comprise plastic, and tolerances for forming plastic parts are typically high relative to the tolerances for the conventional stamping and stitching processes for forming contact members 6. Tolerance stacking, or accumulation, of tolerances of a plurality of components may add to the error introduced by the individual tolerances. Nonplanarities in the seating part 5a, as well as nonplanarities in the IC package substrate 10, for example, bowing, may further contribute to error in the seating plane, and therefore error in the spacing between the seating plane and the plane of the contact arm tips 7.
One example of a problem resulting from improper spacing between the plane of the contact arm tips 7 of a socket 1 and the seating plane of the BGA package 9 is that a trace (not shown) on a bottom surface 8 of the IC package substrate 10 may interfere with the movement of a contact arm 6a, 6b if the seating plane of the IC package is positioned too close to the plane of the contact arm tips 7. The trace may protrude from the bottom surface 8 of the IC package substrate 10. The contact arm tip 7 of a moving contact arm 6a or 6b may intersect the trace, preventing further movement, and therefore, contact with the solder ball 11. If the contact member touches or rubs against the BGA package substrate 10, especially while moving to make contact with the solder ball 11, the bottom surface 8 of the IC package substrate 10 may be scratched, which may result in unreliability of the BGA package 9 in later service due to entry of moisture or other contaminants or undetected damage to circuitry, since scratches may damage the passivation layer on the IC package substrate 10, or may expose the underlying traces and cause shorts.
Another problem with spacing error results from the contact arm tips 7 touching the BGA package 9. The IC package substrate 10 may expand, for example during an increase in temperature, such as burn-in testing. Contact arm tips 7 touching the IC package substrate 10 when the IC package substrate 10 expands may move with the expansion of the IC package substrate 10, causing the contact arms 6a, 6b to pull apart and lose reliable electrical communication with the solder ball 11.
A third concern in relation to BGA package test sockets is that the IC package may not be held in the socket securely enough to maintain a valid testing process through sufficient continuous electrical communication between the socket and the IC package, yet not so securely held that the IC package or its electrical connections are damaged, particularly during removal of the IC package from the test socket.
In view of the foregoing, it appears that a socket with an improved seating plane for an IC package and a method of forming electrical connection with improved accuracy between a socket and an IC package would be useful.